SPATIOTEMPORAL DISPERSAL STRATEGIES AND ANNUAL PLANT-SPECIES COEXISTENCE IN A STRUCTURED LANDSCAPE

Field observations and models have shown: (1) differences in dispersal and dormancy strategies could account for species coexistence in dist urbed environments, (2) small scale disturbances contribute to species coexistence in plant communities, and (3) recolonization success may depend on the spatial pattern of suitable sites for establishment. A s patially explicit, two-species, simulation model was developed to exam ine the interaction between dispersal strategies, dormancy strategies and small-scale disturbances on the pattern of coexistence of two annu al plant species in patchy landscapes. Patchy landscapes with three hi erarchical levers of structure were randomly generated and the dynamic s for species differing in mean dispersal distance and dormancy strate gy was simulated. The results showed that when species differed only i n mean dispersal distance, the species with the shorter range dispersa l would always displace the other. When disturbances prevented reprodu ction and activated dormant seeds, then differences in dormancy strate gy could result in coexistence of species with different dispersal cap abilities. When species differed only in their dormancy fate (i.e., th e proportion of dormant seeds produced per year), the range of disturb ance intensities allowing coexistence was rather narrow: but differenc es in the response of species to disturbance (i.e., disturbance-broken seed dormancy versus gradual germination of dormant seeds) allowed co existence under a wider range of disturbance intensities. These result s suggest that differences in the ways species exploit the resources i n space and time can mediate coexistence through storage effects. The patterns of coexistence obtained in the simulations were consistent wi th conclusions of a field study, in species-rich Mediterranean old-fie lds, about the role for coexistence of the interactions among dispersa l and dormancy strategies, spatial heterogeneity, and small-scale dist urbances. Coexistence patterns depended on the degree of suitability a nd the patchiness of the landscapes, mostly in relation to the interac tions between landscape structure and mean dispersal distance, but als o through complex interactions with dormancy characteristics. Therefor e, disturbance regimes are not sufficient for predicting patterns of c oexistence, and the unique spatial features of the landscapes need to be taken into account.